Trojan Asteroid That Will Follow Earth in Orbit for 4,000 Years Discovered After a Decade of Searching

An International team of astronomers led by researcher Toni Santana-Ros, from the University of Alicante and the Institute of Cosmos Sciences of the University of Barcelona (ICCUB), has confirmed the existence of the second Earth Trojan asteroid known to date, the 2020 XL₅, after a decade of search. The results of the study have been published in the journal Nature Communications.

All celestial objects that roam around our solar system feel the gravitational influence of all the other massive bodies that build it, including the Sun and the planets. If we consider only the Earth-Sun system, Newton’s laws of gravity state that there are five points where all the forces that act upon an object located at that point cancel each other out. These regions are called Lagrangian points, and they are areas of great stability. Earth Trojan asteroids are small bodies that orbit around the L₄ or L₅ Lagrangian points of the Sun-Earth system.

These results confirm that 2020 XL₅ is the second transient Earth Trojan asteroid known to date, and everything indicates it will remain Trojan —that is, it will be located at the Lagrangian point— for four thousand years, thus it is qualified as transient. The researchers have provided an estimation of the object bulk size (around one kilometer in diameter, larger than the Earth Trojan asteroid known to date, the 2010 TK₇, which was 0.3 kilometers in diameter), and have made a study of the impulse a rocket needs to reach the asteroid from Earth.

Lagrange points are places in space where the gravitational forces of two massive bodies, such as the Sun and a planet, balance out, making it easier for a low-mass object (such as a spacecraft or an asteroid) to orbit there. This diagram shows the five Lagrange points for the Earth-Sun system. (The size of Earth and the distances in the illustration are not to scale.) Credit: NOIRLab/NSF/AURA/J. da Silva, Acknowledgment: M. Zamani (NSF’s NOIRLab)

Although Trojan asteroids have been known to exist for decades in other planets such as VenusVenus, the second planet from the sun, is named after the Roman goddess of love and beauty. After the moon, it is the second-brightest natural object in the night sky. Its rotation (243 Earth days) takes longer than its orbit of the Sun (224.7 Earth days). It is sometimes called Earth's "sister planet" because of their similar composition, size, mass, and proximity to the Sun. It has no natural satellites.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>Venus, MarsMars is the second smallest planet in our solar system and the fourth planet from the sun. Iron oxide is prevalent in Mars' surface resulting in its reddish color and its nickname "The Red Planet." Mars' name comes from the Roman god of war.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>Mars, JupiterJupiter is the largest planet in the solar system and the fifth planet from the sun. It is a gas giant with a mass greater then all of the other planets combined. Its name comes from the Roman god Jupiter.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>Jupiter, UranusUranus is the seventh farthest planet from the sun. It has the third-largest diameter and fourth-highest mass of planets in our solar system. It is classified as an "ice giant" like Neptune. Uranus' name comes from a Latinized version of the Greek god of the sky.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>Uranus, and NeptuneNeptune is the farthest planet from the sun. In our solar system, it is the fourth-largest planet by size, and third densest. It is named after the Roman god of the sea.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>Neptune, it was not until 2011 that the first Earth Trojan asteroid was found. The astronomers have described many observational strategies for the detection of new Earth Trojans. “There have been many previous attempts to find Earth Trojans, including in situ surveys such as the search within the L₄ region, carried out by the NASAEstablished in 1958, the National Aeronautics and Space Administration (NASA) is an independent agency of the United States Federal Government that succeeded the National Advisory Committee for Aeronautics (NACA). It is responsible for the civilian space program, as well as aeronautics and aerospace research. It's vision is "To discover and expand knowledge for the benefit of humanity."” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>NASA OSIRIS-Rex spacecraft, or the search within the L₅ region, conducted by the JAXAFormed in 2003, the Japan Aerospace Exploration Agency (JAXA) was born through the merger of three institutions, namely the Institute of Space and Astronautical Science (ISAS), the National Aerospace Laboratory of Japan (NAL) and the National Space Development Agency of Japan (NASDA). JAXA performs various activities related to aerospace, from basic research in the aerospace field to development and utilization and is responsible for research, technology development, and launch of satellites into orbit, and is involved in advanced missions such as asteroid exploration and possible human exploration of the Moon.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>JAXA Hayabusa-2 mission,” notes Toni Santana-Ros, author of the publication. He adds that “all the dedicated efforts had so far failed to discover any new member of this population.”

The low success in these searches can be explained by the geometry of an object orbiting the Earth-Sun L₄ or L₅ as seen from our planet. These objects are usually observable close to the sun. The observation time window between the asteroid rising above the horizon and sunrise is, therefore, very small. Therefore, astronomers point their telescopes very low on the sky where the visibility conditions are at their worst and with the handicap of the imminent sunlight saturating the background light of the images just a few minutes in the observation.

To solve this problem, the team carried out a search of 4-meter telescopes that would be able to observe under such conditions, and they finally obtained the data from the 4.3m Lowel Discovery telescope (Arizona, United States), and the 4.1m SOAR telescope, operated by the National Science Foundation NOIRLab (Cerro Pachón, Chile).

The discovery of the Earth Trojan asteroids is very significant because these can hold a pristine record on the early conditions in the formation of the Solar System, since the primitive trojans might have been co-orbiting the planets during their formation, and they add restrictions to the dynamic evolution of the Solar System. In addition, Earth Trojans are the ideal candidates for potential space missions in the future.

Since the L₄ Lagrangian point shares the same orbit as the Earth, it takes a low change in velocity to be reached. This implies that a spacecraft would need a low energy budget to remain in its shared orbit with the Earth, keeping a fixed distance to it. “Earth Trojans could become ideal bases for an advanced exploration of the Solar System; they could even become a source of resources,” concludes Santana-Ros. 

The discovery of more trojans will enhance our knowledge of the dynamics of these unknown objects and will provide a better understanding of the mechanics that allow them to be transient.

For more on this research, see Existence of Earth Trojan Asteroid Confirmed.

Reference: “Orbital stability analysis and photometric characterization of the second Earth Trojan asteroid 2020 XL₅” by T. Santana-Ros, M. Micheli, L. Faggioli, R. Cennamo, M. Devogèle, A. Alvarez-Candal, D. Oszkiewicz, O. Ramírez, P.-Y. Liu, P. G. Benavidez, A. Campo Bagatin, E. J. Christensen, R. J. Wainscoat, R. Weryk, L. Fraga, C. Briceño and L. Conversi, 1 February 2022, Nature Communications.
DOI: 10.1038/s41467-022-27988-4